You will want to track the movement as if it were a bilateral movement, meaning if you achieved 10 reps on each leg for Bulgarian Split Squats with 20lb dumbbells, you will track it as 10 reps and 20lbs used for the weight. You will not combine the weight and reps together into one number.
If one side of the body is weaker than the other and you get different reps or have to use different weights, then you can write in the “notes” section of that particular exercise what you did for the weaker side so that the following week you know what you did.